In the chemical industry, electrolytic cells are extensively used as apparatus for the production of different basic materials, such as hydrogen, oxygen, chlorine, alkalimetal hydroxides, chlorates, chlorites and others. Different types of electrolytic cells are in commercial use, for instance cells with horizontal or vertical anodes; cells with just one anode and cathode compartment, the so-called monopolar cells; as well as cells with a plurality of anode and cathode compartments electrically connected in series, the so-called bipolar cells.
Due to the limited product capacity of an individual cell, commercial plants normally comprise a plurality of electrolytic cells electrically connected in series, forming the so-called cell circuit. For instance, a chlor-alkali cell circuit, used for production of chlorine, hydrogen and alkali hydroxides, may comprise 50 to 100 cells or even more. Such circuits are equipped with intercell connectors between all cells, thus assuring a perfect current flow through the total circuit. The intercell connectors are made from highly conductive metals, for instance copper or aluminum. Each connector is attached to the cathode lead-out of one cell and the anode lead-in of the adjacent cell.
In many cases, but especially in the case of chlor-alkali cells, the life time of certain cell parts, such as anodes, diaphragms or other cell internals, is limited. This fact requires the removal of individual cells from time to time, and transportation to a cell workshop for replacement of the spent or exhausted cell parts. Normally such cell circuits are equipped with portable jumper switches for bypassing the electrical current around each incapacitated cell to the two adjacent cells, thus allowing steady operation of the cell circuit without any interruptions due to the incapacity of an individual cell.
Before removal of an incapacitated cell, the terminals of the jumper switch have to be connected to the cathode lead-out of one and the anode lead-in of the other adjacent cell. Then, by operating the switch, the incapacitated cell can be shut down and afterwards electrically disconnected from the two adjacent cells. This disconnection is performed by removal of the intercell connectors, the weight of each of them has to be limited. Therefore, for cells with high current rate, a plurality of single intercell connectors has to be provided. Normally these individual intercell connectors are disposed across the length of the gap between the cells. For removal or installation of the intercell connectors, this gap must have a certain clearance, allowing an operator to work inside it.
The necessity of such operation gaps between all cells is a certain disadvantage of conventional circuits of monopolar cells, especially when compared with circuits of bipolar cells. Bipolar cells comprise a plurality of anode and cathode compartments electrically connected in series, wherein the connection of adjacent anode and cathode compartments is executed by direct attachment of the backsides of these compartments. Thus, circuits of bipolar cells only need intercell connectors and operation gaps between the end compartment of one and the first compartment of the adjacent cell, and it is evident that the amount of material for intercell connectors, and the floor space requirement, are much lower than for circuits of monopolar cells for the same product capacity. For instance, a circuit of monopolar cells may comprise 100 cells operated at 100 kA. A circuit of bipolar cells for the same product capacity and operated at 100 kA may comprise 10 cells, each with 10 anode and 10 cathode compartments. In both cases the cells may be installed in two parallel rows, each row with one half the total number of cells. Consequently, the monopolar cell circuit comprises 98 operation gaps between the cells, whereas the bipolar cell circuit comprises eight operation gaps only. Thus, the amount of material for the inter-cell connectors of the monopolar cells is about 12 times higher than for the bipolar cells and the extension of the monopolar cell rows is at least 20 meters longer.
On the other hand, following this example, each bipolar cell has 10 times higher product rate, 10 times higher voltage drop and about 10 times higher weight than each monopolar cell. Due to these facts, different disadvantages occur for circuits of bipolar cells in cases where removal of individual cells during continuous operation of the circuit is necessary, as for instance in the chloralkaline electrolysis. During removal of a bipolar cell, the on-stream factor of the circuit is just 90%, compared to 99% during removal of a monopolar cell. The jumper switch of the bipolar cell circuit has to be designed for ten times higher voltage drop. The transport means for moving the bipolar cells between cell circuit and cell workshop, as well as the cell workshop itself, have to be designed in regard to the extremely high weight and volumes of these bipolar cells. Furthermore, construction of a bipolar cell is much more complicated than that of a monopolar cell and may result in more effort for maintenance and repair.
These disadvantages may be the main reasons that monopolar cell circuits dominate the important field of chloralkaline electrolysis.